The combination of microarray analysis of gene expression and molecular studies of transcription-factor activity are beginning to reveal the circuitry of gene expression networks. We have shown that the myogenic transcription factor, Myod, functions in an instructive chromatin context and directly regulates genes that are expressed throughout the myogenic program, achieving promotor-specific regulation of its own binding and activity through a feed-forward mechanism. This gives us the opportunity to deconstruct the myogenic program into discrete molecular regulatory phases, similar to the cellular compartments identified in hematopoietic cell development. The broad and long-term objective is to apply the knowledge we have gained from our studies of gene regulation in myogenesis to identify the molecular defect(s) in the differentiation program of rhabdomyosarcomas. We will use our model systems of Myod-mediated regulation of gene expression to test the broad hypothesis that rhabdomyosarcomas have inactivated a discrete subprogram, or subprograms, maintaining early aspects of Myod-mediated regulative growth and migration, but inactivating later aspects of terminal differentiation and cell cycle withdrawal. The specific aims will: (1) test the hypothesis that Msc promotes myoblast growth by activating an initial subset of skeletal muscle genes and suppressing genes that mediate terminal differentiation; (2) determine whether Msc and E2A-AD1 regulate a subset of bHLH target genes that keep rhabdomyosarcoma cells in a state of regulative growth and prevent Myod-mediated terminal differentiation; and (3) determine the role of micro-RNA in the transition from regulative growth to differentiation and test the hypothesis that bHLH induction of micro-RNAs is necessary to inhibit regulative growth and initiate terminal differentiation in normal myogenesis and in rhabdomyosarcomas. PUBLIC HEALTH RELEVENCE: The significance of the application is that it will identify the molecular regulation of the transition from a growth phase to a differentiation phase in normal development and cancer. The health relatedness is that identification of the molecular steps in this transition from a growth phase to a differentiation phase will provide new strategies for developing cancer therapeutics based on cell differentiation. The model biological system of skeletal muscle differentiation has permitted the analysis of how a complex program of gene expression is regulated on a molecular level. The opportunity now exists to use this knowledge to identify aspects of cell differentiation that are not accurately executed in cancer cells, and in rhabdomyosarcoma cells in particular. The knowledge from these studies will identify new therapeutic targets and strategies for the development of novel, differentiation based therapies.